BYSTANDER TUMORICIDAL EFFECT IN THE TREATMENT OF EXPERIMENTAL BRAIN-TUMORS

THE RETROVIRUS-MEDIATED TRANSFER of the herpes simplex virus-thymidine kinase (HSV-tk) gene into tumor cells renders them sensitive to the c ytocidal effect of the antiviral drug ganciclovir. This method has sho wn promising results as a treatment for experimental brain tumors. The se experiments indicate that a major mechanism for the effectiveness o f HSV-tk retroviral gene therapy may be the bystander tumoricidal effe ct. The bystander effect was hypothesized to explain tumor eradication , given that the efficacy of in vivo gene transfer to tumor cells was less than 100%. We demonstrate, in this report, that the bystander tum oricidal effect is a major contributor to the tumoricidal effect of ga nciclovir in cell culture experiments using the mouse K1735 C19 cerebr al melanoma line, thereby expanding the observation of the bystander p henomenon to a broader range of tumor types. The bystander effect was studied in vitro by coculturing wild-type C19 melanoma cells with HSV- tk-expressing C19 (C19-STK) cells. A maximal tumoricidal effect was se en when only 1 in 10 tumor cells expressed the HSV-tk gene. This sugge sts that in effect, 1 tumor cell with the HSV-tk gene, when given ganc iclovir, will destroy 10 neighboring or bystander cells. The destructi on of bystander cells does not appear to be mediated by a soluble fact or(s) released into the media but, rather, requires close cell proximi ty or cell contact. In addition, HSV-tk-expressing C19 cells can exert an antitumoral effect not only on wild-type C19 cells but also on cel ls from a variety of different tumor cell lines, including a human gli oblastoma multiforme cell line, indicating that the bystander effect i s not a cell line-specific phenomenon. Finally, we observed that the b ystander tumoricidal effect could be harnessed directly without using retrovirus-producing cells to increase survival in the mouse C19 brain tumor model. The potential implications of our findings in treating h uman brain tumors are discussed.